Atomic Force Microscopy (AFM) is a well known form of Scanning Probe Microscopy (SPM). AFM is conventionally employed in range of surface imaging, nanometer scale analysis and manipulation applications. Imaging is achieved via AFM by detecting forces occurring between a sensor in form of a tip at one end of a cantilever and a sample to be imaged. The tip and cantilever in combination will be herein after referred to as a tip assembly in the interests of convenience. The tip is a relatively reliable tool for providing highly localized confinement of interaction. This property has opened up a range of applications in the nanotechnology field. For example, in “The “Millipede”—More than one thousand tips for future AFM data storage”, P. Vettiger et al, IBM Journal of Research and Development. Vol.44 No.3, May 2000, there is described a data storage device based on AFM probe technology providing smaller form factor, higher capacity, lower power, and lower cost than conventional memory devices. The storage density achievable in such a storage device depends on the durability and quality of the tip. It would therefore be desirable to provide such tips with optimal durability and quality via a cost effective process. It would also be desirable to produce highly integrated arrays of such sensors.
In a conventional AFM instrument, the tip assembly, which is responsible for the spatial resolution of the instrument, contributes a significant portion of the total cost of ownership (TC0) of the instrument. The cantilever of the tip assembly is typically hand-fabricated or batch microfabricated cantilever. In situ within the instrument, one end of the cantilever is fixed and the other remains free. The tip is located at the free end. In operation, the cantilever permits sensing of a force interaction between the tip and the surface of the probed sample. A surface observation made by such an instrument is a function of the sample surface topography and the shape of the tip. To minimize noise, it is desirable not only to make the tip as sharp as possible, but also to make the aspect ratio of the tip as high as possible. The aspect ratio of the tip is an inverse function of the cone angle of the tip. However, as the aspect ratio of a tip is increased, so the tip becomes more fragile and subject to wear. Usually, the shape of the tip is determined by way of tradeoff between the tip robustness and the imaging quality.
An example of a conventional tip assembly comprises a microfabricated cantilever with an integrated silicon tip. While allowing for some economy of scale in production costs over hand-fabricated cantilevers, this technology is nonetheless still relatively expensive. In addition, the silicon tip is subject to wear during normal operation. Wearing of the tip can lead to inconsistent imaging results. Wearing of the tip also makes the tip a consumable of the instrument, introducing a need for regular tip monitoring and replacement time costs. It would be desirable to reduce such monitoring and tip replacement requirements.
WO 99/56176 discloses a method of manufacturing a tipped cantilever comprising forming a tip-like indent in a substrate, depositing a photoresist layer which fills the tip-like indent and covers at least a part of said substrate, and photolithographically structuring the photoresist layer to form the tipped cantilever with tip out of the photoresist. This technique lends itself well to batch microfabrication of plastic cantilevers at a lower cost than the aforementioned silicon technology.
In Review of Scientific Instruments, vol. 70, no. 5, May 1999, pages 2398-2401, G. Genolet et al., “Soft, entirely photoplastic probes for scanning force microscopy”, there is disclosed a probe made entirely of plastic material for scanning probe microscopy. A polymer is used for forming the cantilever. The polymer provides mechanical properties that are difficult to achieve with classical silicon technology. The fabrication process is a batch process in which the integrated tip and the lever are defined in one photolithography step.
Applied Physical Letters, vol. 77, no. 21, 20 Nov. 2000, R. Stevens et al., “Improved fabrication approach for carbon nanotube probe devices”, discloses an improved process for simple and efficient fabrication of carbon nanotube probe devices. The process requires two steps. First, a nanotube cartridge is created using chemical vapor deposition. Then, the nanotubes are transferred from the cartridge to a device using an electric field.
Carbon nanotube mounted Si and/or silicon nitride tips are commercially available, for example, from PIEZOMAX Technologies Inc. The nanotube is grown and then mounted on such tips manually. The length of the attached nanotube is then iteratively tuned via a series of alternating shortening and re-inspection steps. It will be appreciated that this serial production process is both time consuming, expensive, and unsuitable for batch fabrication. Additionally, the robustness of the tip assembly is governed by the bond between the nanotube and the tip.
In general, the quality and durability of tip assemblies produced according to the aforementioned conventional techniques present limitations is less than optimal for the range of applications now envisaged for such instruments. Furthermore, the reproducibility of such tip assemblies via conventional methods is difficult to achieve without incurring additional cost. It would desirable therefore, as mentioned earlier, to provide microstructures such as tip assemblies of optimal quality, durability and versatility that can be manufactured cheaply.